In physics class, the girl to boy ratio is $9$ to $5$. If there are a total of $56$ students, how many boys are there?
A ratio of $9$ girls to $5$ boys means that a set of $14$ students will have $9$ girls and $5$ boys. A class of $56$ students has $4$ sets of $14$ students. Because we know that there are $5$ boys in each set of $14$ students, the class must have $4$ groups of $5$ boys each. There is a total of $20$ boys in physics class.